Quiz 13.2: “Concentration and Molarity” You may use your notes and your book to complete the quiz on your own… Use the force!
Solubility and Polarity Solubility: the ability of one substance to dissolve into another at a given temperature and pressure; expressed in terms of the amount of solute that will dissolve in a given amount of solvent to produce a saturated solution. Polar compounds tend to dissolve in other polar compounds, and non-polar compounds tend to dissolve in other non-polar compounds.
Vitamin C is a Water-Soluble Vitamin Vitamin C has several −OH groups. These −OH groups form strong hydrogen bonds with the −OH groups in water, so vitamin C is very soluble in water. It is not possible to overdose on vitamin C because excess vitamin C is removed by the kidneys and does not build up in your body.
Vitamin A is a Fat-Soluble Vitamin Vitamin A has a long, non-polar carbon-hydrogen chain. Its non-polarity makes it very soluble in fats and oils, which are also non-polar, but NOT in water. It is possible to overdose on vitamin A because it can build up in your body fat.
The Rule Is “Like Dissolves Like” Polar molecules will dissolve in polar solvents. Example: water and ethanol (both polar) Non-polar molecules will dissolve in non-polar solvents. Example: oil and dry-cleaning fluid (both non-polar) Like Dissolves Like
Miscible/Immiscible Miscible: describes two or more liquids that are able to dissolve into each other in various proportions. Example: Water and ethanol (both polar) Immiscible: describes two or more liquids that do not mix with each other Example: Water and benzene (1 polar/1 non-polar)
Solubilities of Solid Compounds The only place where dissolving can take place is at the surface where solute and solvent molecules are in contact. Greater surface area speeds up the dissolving process. If a solid has been broken into small particles, the surface area is much greater and the rate of the dissolving process is increased.
Solubilities of Solids Generally Increase with Temperature Another way to make most solids dissolve more and faster is to increase the temperature. Increasing the temperature is effective because, in general, solvent molecules with greater kinetic energy can dissolve more solute particles.
Both Enthalpy and Entropy Affect the Solubility of Salts Until now, we have not made a distinction between the dissolving process of a covalent solid (sugar) and an ionic solid (NaCl). Surface area and temperature both affect both covalent and ionic solids. However, the dissolving process of an ionic compound involves a unique factor: the separation of ions from the crystal lattice into individual dissolved ions…
Solubilities of Ionic Compounds (Salts) This process is called dissociation. Dissociation: the separating of a molecule into simpler molecules, atoms, radicals, or ions. NaCl (s) → Na + (aq) + Cl − (aq)
Hydration If water is the solvent, dissociation also involves hydration. Hydration: the strong affinity of water molecules for particles of dissolved or suspended substances that causes electrolytic dissociation What? This means that the water molecules surround the dissociated ions.
Enthalpy Affects the Solubility of Salts It takes a lot of energy to separate the ions from the crystal, so ΔH is positive. As the polar ends of the water molecules approach the ions, energy is released, so ΔH is negative.
Entropy Affects the Solubility of Salts Entropy increases as the ions are scattered throughout the solution, so ΔS is positive. The entropy decreases as the water molecules are structured around the ions, so ΔS is negative. The net result of all the enthalpy and entropy changes determine the solubility of an ionic solid.
Solubilities of Ionic Compounds Solubilities of ionic compounds are difficult to predict because of the many factors involved. They must be measured experimentally. From experimental results of ionic solubilities in water, some patterns emerge. Categories such as soluble and insoluble can be useful in many cases.
Solubility Rules for Some Common Ionic Compounds
Saturation Saturated solution: A solution that cannot dissolve any more solute under the given conditions. In a saturated solution, some excess solute can remain undissolved. The mass that dissolves is equal to the solubility value for that temperature. Unsaturated solution: a solution that contains less solute than a saturated solution and that is able to dissolve additional solute.
Mass of Solute Added Versus Mass of Solute Dissolved
Solubility can be Exceeded Supersaturated solution: a solution holding more dissolved solute than what is required to reach equilibrium at a given temperature. Supersaturated solutions have more solute dissolved than the solubility indicates would normally be possible, but only as long as there is no excess undissolved solute remaining.
Solubility Equilibrium In a saturated solution, the solute is recrystallizing at the same rate that it is dissolving.
Solubility Equilibrium It is a state of dynamic equilibrium. There is constant exchange, yet there is no net change. Solubility equilibrium: the physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates.
Gases Can Dissolve in Liquids Gas solubility depends on pressure and temperature. In a gas, there is low attraction between the molecules. Henry’s law: at constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas on the surface of the liquid. Pressure Solubility
Temperature Also Affects Gas Solubility Gases are less soluble in liquids of higher temperature because the increased molecular motion in the solution allows gas molecules to escape their loose association with the solvent molecules.
Homework Section review, pg. 477: #1-10 Concept review: “Solubility and the Dissolving Process” You will have a quiz over this section next time…